Method and apparatus for detecting a voice activity in an input audio signal

ABSTRACT

The disclosure provides a method and an apparatus for detecting a voice activity in an input audio signal composed of frames. A noise characteristic of the input signal is determined based on a received frame of the input audio signal. A voice activity detection (VAD) parameter is derived based on the noise characteristic of the input audio signal using an adaptive function. The derived VAD parameter is compared with a threshold value to provide a voice activity detection decision. The input audio signal is processed according to the voice activity detection decision.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 15/157,424, filed on May 18, 2016, which is a continuation of U.S. patent application Ser. No. 13/891,198, filed on May 10, 2013, now U.S. Pat. No. 9,368,112. The U.S. patent application Ser. No. 13/891,198 is a continuation of International Application No. PCT/CN2010/080227, filed on Dec. 24, 2010. The afore-mentioned patent applications are hereby incorporated by reference in their entireties.

TECHNICAL FIELD

The disclosure relates to a method and an apparatus for adaptively detecting a voice activity in an input audio signal composed of frames, and in particular to a voice activity detection method and apparatus using non-linearly processed sub-band segmental signal to noise ratio parameters.

BACKGROUND

Voice activity detection (VAD) is generally a technique for detecting a voice activity in a signal. Voice activity detectors are widely used in the telecommunication field. A basic function of a voice activity detector is to detect, in communication channels, the presence or absence of active signals, such as speech or music signals. The voice activity detector can be provided within a communication network, wherein the network can decide to compress transmission bandwidth in periods where active signals are absent, or to perform other processing depending on a voice activity detection decision (VADD) indicating whether there is an active signal or not. A voice activity detector can compare a feature parameter or a set of feature parameters extracted from the input signal to corresponding threshold values, and determine whether the input signal includes an active signal or not based on the comparison result. The performance of a voice activity detector depends to a high degree on the choice of the used feature parameters.

There have been many feature parameters proposed for voice activity detection, such as energy based parameters, spectral envelope based parameters, entropy based parameters, higher order statistics based parameters and so on. In general, energy based parameters provide a good voice activity detection performance. In recent years, sub-band signal to noise ratio (SNR) based parameters as a kind of energy based parameters have been widely used in the telecommunication field. In sub-band SNR based voice activity detectors, the SNR for each frequency sub-band of an input frame is detected, and the SNRs of all sub-bands are added to provide a segmental SNR (SSNR). The SSNR can be compared with a threshold value to make a voice activity detection decision (VADD). The used threshold is usually a variable, which is adaptive to a long term SNR (LSNR) of the input signal or a level of background noise.

In a recently completed ITU-T (International Telecommunication Union Telecommunication Standardization Sector) Recommendation G.720.1 (G.720.1 hereinafter), the conventional SSNR parameter has been improved by applying a non-linear processing to get a modified SSNR (MSSNR). The calculated MSSNR is also compared to a threshold which is determined from a threshold table according to the LSNR of the input signal, the background noise variation and the voice activity detection (VAD) operating point, where the VAD operating point defines the tradeoff of the VAD decision between active and inactive detection, for example a quality-preferred operating point will make the VAD favor the detection of active signals and vice versa.

Although the MSSNR parameter used by G.720.1 does increase the performance of the voice activity detection, the VAD performance in a non-stationary and low SNR background environment still needs improvement. Conventional voice activity detectors are designed to balance their performances in various background noise conditions. Accordingly, conventional voice activity detectors have a performance which is sub-optimal for specific conditions and in particular in a non-stationary and low SNR background environment.

SUMMARY

The disclosure provides, according to a first aspect, a method for adaptively detecting a voice activity in an input audio signal. The input audio signal is composed of frames. The method includes the following:

determining a noise characteristic of the input signal based at least on a received frame of the input audio signal;

deriving a VAD parameter adapted to or selected dependent on the noise characteristic of the input audio signal; and

comparing the derived VAD parameter with a threshold value to provide a voice activity detection decision.

Implementation forms of the first aspect may use energy based parameters, spectral envelope based parameters, entropy based parameters or higher order statistics based parameters as VAD parameters.

In a possible implementation of the first aspect of the present disclosure, a method for adaptively detecting a voice activity in an input audio signal, which is composed of frames, comprises the steps of:

determining a noise characteristic of the input signal based at least on a received frame of the input audio signal;

dividing the received frame of the input audio signal into several sub-bands;

obtaining a signal to noise ratio (SNR) for each s of the received frame;

for each sub-band, calculating a sub-band specific parameter based on the respective sub-band's SNR using an adaptive function, wherein at least one parameter of the adaptive function is selected dependent on the noise characteristic of the input audio signal;

deriving a modified segmental SNR as a VAD parameter by adding the sub-band specific parameter of each sub-band; and

comparing the derived modified segmental SNR with a threshold value to provide a VAD decision.

In a possible implementation of the first aspect of the present disclosure, the determined noise characteristic of the input audio signal is formed by a long term SNR of the input audio signal.

In a further possible implementation of the first aspect of the present disclosure, the determined noise characteristic of the input audio signal is formed by a background noise variation of the input audio signal.

In a still further possible implementation of the first aspect of the present disclosure, the determined noise characteristic of the input audio signal is formed by a combination of the long term SNR and the background noise variation of the input audio signal.

In an implementation of the first aspect of the present disclosure, the adaptive function used for calculating the sub-band specific parameter is formed by a non-linear function.

In a possible implementation of the method for adaptively detecting a voice activity in an input audio signal according to the first aspect of the present disclosure, a SNR for each sub-band of the input frame is obtained by obtaining a signal energy for each sub-band, e.g. a signal energy for each sub-band of the input frame.

In a further possible implementation of the method for adaptively detecting a voice activity in an input audio signal according to the first aspect of the present disclosure, the SNR for each sub-band of the input frame is obtained by estimating a background noise energy for each sub-band.

In a further possible implementation of the method for adaptively detecting a voice activity in an input audio signal according to the first aspect of the present disclosure, the SNR for each sub-band of the input frame is obtained by calculating the SNR for each sub-band depending on the signal energy and the background noise energy of the respective sub-band.

In a further possible implementation of the method for adaptively detecting a voice activity in an input audio signal according to the first aspect of the present disclosure, the modified SSNR (mssnr) is calculated by adding sub-band specific parameters (sbsp) as follows:

${mssnr} = {\sum\limits_{i = 0}^{N}{{sbsp}(i)}}$

In a possible implementation of the method for adaptively detecting a voice activity in an input audio signal according to the first aspect of the present disclosure, the modified segmental SNR (mssnr) is calculated as follows:

${mssnr} = {\sum\limits_{i = 0}^{N}\left( {{f\left( {{snr}(i)} \right)} + \alpha} \right)^{\beta}}$ wherein snr(i) is a SNR of the i^(th) sub-band of the input frame, N is the number of frequency sub-bands into which the input frame is divided, (f(snr(i))+α)^(β) is the adaptive function (AF) used to calculate the sub-band specific parameter sbsp(i), and α, β are two configurable variables of the adaptive function (AF).

In a possible implementation of the method for adaptively detecting a voice activity in an input audio signal according to the first aspect of the present disclosure, the first variable α of the adaptive function (AF) may depend on a long term SNR (lsnr) of the input audio signal as follows: α=g(i, lsnr) wherein g(i, lsnr) is a linear or non-linear function, and the second variable β of the adaptive function (AF) may depend on the long term SNR (lsnr) and φ as follows: β=h(lsnr, φ) wherein h(lsnr, φ) is a non-linear function and φ=f(snr(i))+α.

In a further implementation of the method for adaptively detecting a voice activity in an input audio signal according to the first aspect of the present disclosure, the first variable α of the adaptive function (AF) may be calculated by: α=g(i, lnsr)=α(i)lsnr+b(i) wherein a(i), b(i) are real numbers depending on a sub-band index i, and the second variable β of the adaptive function (AF) may be calculated by:

$\beta = {{h\left( {{lsnr},\varphi} \right)} = \left\{ \begin{matrix} \beta_{1} & {\varphi \geq {d\mspace{14mu}{and}\mspace{14mu}{lsnr}} \geq e_{2}} \\ \beta_{2} & {\varphi \geq {d\mspace{14mu}{and}\mspace{14mu} e_{1}} < {lsnr} \leq e_{2}} \\ \beta_{3} & {\varphi \geq {d\mspace{14mu}{and}\mspace{14mu}{lsnr}} \leq e_{1}} \\ \beta_{4} & {otherwise} \end{matrix} \right.}$ wherein β₁<β₂<β₃ and β₄ and d and e₁<e₂ are integer or floating numbers, and lsnr is the long term SNR of the input audio signal.

In a possible implementation of the method for adaptively detecting a voice activity in an input audio signal according to the first aspect of the present disclosure, the derived modified segmental SNR (mssnr) is compared with a threshold value (thr) being set to:

${thr} = \left\{ \begin{matrix} k_{1} & {{lsnr} > e_{2}} \\ k_{2} & {e_{1} < {lsnr} \leq e_{2}} \\ k_{3} & {{lsnr} \leq e_{1}} \end{matrix} \right.$ wherein k₁>k₂>k₃ and e₁<e₂ are integer or floating numbers, and the voice activity detection decision (VADD) is generated as follows:

${V\; A\; D\; D} = \left\{ \begin{matrix} 1 & {{mssnr} > {thr}} \\ 0 & {{mssnr} \leq {thr}} \end{matrix} \right.$ wherein VADD=1 indicates an active frame with voice activity being present, and VADD=0 indicates a passive frame with voice activity being absent.

In a possible implementation of the method for adaptively detecting a voice activity input audio signal according to the first aspect of the present disclosure, the first variable α of the adaptive function (AF) may be calculated by: α=g(i, lsnr, ε)=α(i)lsnr+b(i)+c(e) wherein a(i), b(i) are real numbers depending on a sub-band index i, and c(ε) is a real number depending on the estimated fluctuation of the background noise of the input audio signal, and the second variable β of the adaptive function (AF) may be calculated by:

$\beta = {{h\left( {{lsnr},\varphi,ɛ} \right)} = \left\{ \begin{matrix} \beta_{1} & {\varphi \geq {d\mspace{14mu}{and}\mspace{14mu}{lsnr}} > {e_{2}\mspace{14mu}{and}\mspace{14mu} ɛ} \leq p} \\ \beta_{2} & {\varphi \geq {d\mspace{14mu}{and}\mspace{14mu}{lsnr}} > {e_{2}\mspace{14mu}{and}\mspace{14mu} ɛ} > p} \\ \beta_{3} & {\varphi \geq {d\mspace{14mu}{and}\mspace{14mu} e_{1}} < {lsnr} < {e_{2}\mspace{14mu}{and}\mspace{14mu} ɛ} \leq p} \\ \beta_{4} & {\varphi \geq {d\mspace{14mu}{and}\mspace{14mu} e_{1}} < {lsnr} < {e_{2}\mspace{14mu}{and}\mspace{14mu} ɛ} > p} \\ \beta_{5} & {\varphi \geq {d\mspace{14mu}{and}\mspace{14mu}{lsnr}} \leq {e_{1}\mspace{14mu}{and}\mspace{14mu} ɛ} \leq p} \\ \beta_{6} & {\varphi \geq {d\mspace{14mu}{and}\mspace{14mu}{lsnr}} \leq {e_{1}\mspace{14mu}{and}\mspace{14mu} ɛ} > p} \\ \beta_{4} & {\varphi < d} \end{matrix} \right.}$ wherein φ=f(snr(i))+α, ε is the estimated fluctuation of the background noise, and d and e₁<e₂ and p are integer or floating numbers.

In a possible implementation of a method for adaptively detecting a voice activity in an input audio signal according to the first aspect of the present disclosure, the derived modified segmental SNR (mssnr) is compared with a threshold value (thr) being set to:

${thr} = \left\{ \begin{matrix} {q_{1} + {r_{1} \cdot {{Min}\left\lbrack {\frac{{lsnr} - v_{1}}{W_{1}},1} \right\rbrack}}} & {{lsnr} > e_{2}} \\ {q_{2} + {r_{2} \cdot {{Min}\left\lbrack {\frac{{lsnr} - v_{2}}{W_{2}},1} \right\rbrack}}} & {e_{1} < {lsnr} \leq e_{2}} \\ {q_{3} + {r_{3} \cdot {{Max}\left\lbrack {\frac{{lsnr} - v_{3}}{W_{3}},1} \right\rbrack}}} & {{lsnr} \leq e_{1}} \end{matrix} \right.$ wherein q₁, q₂, q₃ and r₁, r₂, r₃ and e₁<e₂ and v₁, v₂, v₃ and W₁, W₂, W₃ are integer or floating numbers, and the voice activity detection decision (VADD) is generated as follows:

${V\; A\; D\; D} = \left\{ \begin{matrix} 1 & {{mssnr} > {thr}} \\ 0 & {{mssnr} \leq {thr}} \end{matrix} \right.$ wherein VADD=1 indicates an active frame with voice activity being present, and VADD=0 indicates a passive frame with voice activity being absent.

The disclosure further provides according to a second aspect a voice activity detection (VAD) apparatus for detecting a voice activity in an input audio signal composed of frames.

The VAD apparatus comprises:

-   -   a VAD parameter calculation unit, configured to calculate a         signal to noise ratio (SNR) for each sub-band of a received         frame of the input audio signal, calculate a sub-band specific         parameter based on the respective sub-band SNR for each sub-band         using an adaptive function (AF), and derive a modified segmental         SNR by adding the sub-band specific parameter of each sub-band;         and     -   a VAD decision generation unit, configured to generate a VAD         decision (VADD) by comparing the modified segmental SNR with a         threshold value.

In a possible implementation of the VAD apparatus according to the second aspect of the present disclosure, the apparatus further comprises a noise characteristic determination unit which determines a noise characteristic of the input audio signal based at least on a received frame of the input audio signal.

In a possible implementation of the VAD apparatus according to the second aspect of the present disclosure, the noise characteristic determination unit comprises a long term SNR estimation unit, configured to calculate a long term SNR of the input audio signal.

In a further possible implementation of the VAD apparatus according to the second aspect of the present disclosure, the noise characteristic determination unit comprises a background noise variation estimation unit, configured to calculate a stationarity or fluctuation of the background noise of the input audio signal.

In a further possible implementation of the VAD apparatus according to the second aspect of the present disclosure, the noise characteristic determination unit comprises a long term SNR estimation unit, configured to calculate a long term SNR of the input audio signal, and a background noise variation estimation unit, configured to calculate a stationarity or fluctuation of the background noise of the input audio signal.

In a further possible implementation of the VAD apparatus according to the second aspect of the present disclosure, the adaptive function (AF) is selected dependent on at least one noise characteristic determined by the noise characteristic determination unit.

The disclosure further provides an audio signal processing device according to a third aspect of the present disclosure, wherein the audio signal processing device comprises an audio signal processing unit for processing an audio input signal depending on a VAD decision (VADD) provided by the VAD apparatus according to the second aspect of the present disclosure.

BRIEF DESCRIPTION OF DRAWINGS

In the following, possible implementations of different aspects of the present disclosure are described with reference to the enclosed figures in more detail.

FIG. 1 shows a flow chart of a method for adaptively detecting a voice activity in an input audio signal according to a first aspect of the present disclosure;

FIG. 2 shows a simplified block diagram of a voice activity detection (VAD) apparatus for detecting a voice activity in an input audio signal according to a second aspect of the present disclosure; and

FIG. 3 shows a simplified block diagram of an audio signal processing device according to a third aspect of the present disclosure.

DETAILED DESCRIPTION

FIG. 1 shows a flow chart of a possible implementation of a method for adaptively detecting a voice activity in an input audio signal, according to a first aspect of the present disclosure.

The input audio signal is composed of signal frames. In a first step S1, a noise characteristic of the input audio signal is determined based at least on a received frame of the input audio signal.

In a possible implementation, the input signal is segmented into frames of a predetermined length of e.g. 20 ms, and is inputted frame by frame. In other implementations, the length of the input frame may vary. The noise characteristic of the input audio signal determined in the step S1 may be a long term signal to noise ratio (LSNR) calculated by a LSNR estimation unit. In another possible implementation, the noise characteristic determined in the step S1 is formed by a background noise variation, calculated by a background noise variation estimation unit which calculates a stationarity or fluctuation ε of the background noise of the input audio signal. It is also possible that the noise characteristic determined in the step S1 includes both the LSNR and the background noise variation.

In a further step S2, the received frame of the input audio signal is divided into several frequency sub-bands.

In a further step S3, a sub-band specific parameter is calculated for each of the sub-bands based on the signal to noise ratio (SNR) of each sub-band using an adaptive function (AF).

In a possible implementation, a power spectrum is obtained for each input frame through a fast Fourier transformation (FFT), and the obtained power spectrum is divided into a predetermined number of sub-bands with non-linear widths. Energies for each sub-band are calculated, wherein the energy for each sub-band of the input frame can in a possible implementation be formed by a smoothed energy that is formed by a weighted average of the energies for the same sub-band between the input frame and at least one previous frame. In a possible implementation of the first aspect of the present disclosure, the sub-band SNR of i^(th) sub-band (snr(i)) can be calculated as the modified logarithmic SNR of the frequency sub-band:

${s\; n\;{r(i)}} = {\log_{10}\left( \frac{E(i)}{E_{n}(i)} \right)}$ wherein E(i) is the energy of i^(th) sub-band of the input frame, and E_(n)(i) is the estimated background noise energy of the i^(th) sub-band. The estimated background noise can be calculated by a background noise estimation unit where the estimated energy of each sub-band of the background noise is calculated by moving-averaging the energies of each sub-band among background noise frames detected. This can be expressed as: E _(n)(i)=λ·E _(n)(i)+(1−λ)·E(i) where E(i) is the energy of the i^(th) sub-band of the frame detected as background noise, λ is a “forgetting factor” usually in a range between 0.9-0.99.

After having obtained a SNR (snr) for each sub-band of the input frame in step S3, a sub-band specific parameter (sbsp) is calculated in step S4 based on the respective SNR (snr) of the respective sub-band using an adaptive function (AF). In a possible implementation of the method for adaptively detecting a voice activity, in an input audio signal, at least one parameter of the adaptive function (AF) is selected dependent of the determined noise characteristic of the input audio signal. The noise characteristic determined in step S1 can comprise a long term SNR and/or a background noise variation of the input audio signal. The adaptive function (AF) is a non-linear function.

In a possible implementation of the method for adaptively detecting a voice activity in an input audio signal according to the first aspect of the present disclosure, in step S5, a modified segmental SNR (mssnr) is derived by adding the sub-band's specific parameters (sbsp) as follows:

${mssnr} = {\sum\limits_{i = 0}^{N}{{sbsp}(i)}}$ wherein N is the number of frequency sub-bands into which the input frame is divided, and sbsp(i) is a sub-band specific parameter calculated based on the sub-band's SNR for each sub-band using the adaptive function (AF).

In a possible implementation of the first aspect of the present disclosure, the modified segmental SNR (mssnr) is calculated as follows:

${mssnr} = {\sum\limits_{i = 0}^{N}\left( {{f\left( {{snr}(i)} \right)} + \alpha} \right)^{\beta}}$

-   -   wherein snr(i) is the SNR of the i^(th) sub-band of the input         frame, N is the number of frequency sub-bands into which the         input frame is divided and AF=(f(snr(i))+α)^(β) is the adaptive         function used to calculate the sub-band specific parameter         sbsp(i), and α, β are two configurable variables of the adaptive         function (AF).

In a possible implementation of the first aspect of the present disclosure, the first variable α of the adaptive function (AF) depends on a long term SNR (lsnr) of the input audio signal as follows: α=g(i, lsnr) wherein g(i, lsnr) is a linear or non-linear function,

-   -   and the second variable β of the adaptive function (AF) depends         on the long term SNR (lsnr) and a value φ as follows:         β=h(lsnr, φ)         wherein h(lsnr, φ) is a non-linear function and         φ=f(snr(i))+α

In a possible implementation of the method according to the first aspect of the present disclosure, the first variable α of the adaptive function (AF) may be calculated by: α=g(i, lnsr)=a(i)lsnr+b(i) wherein a(i), b(i) are real numbers depending on a sub-band index i,

-   -   and the second variable β of the adaptive function (AF) may be         calculated by:

$\beta = {{h\left( {{lsnr},\varphi} \right)} = \left\{ \begin{matrix} \beta_{1} & {\varphi \geq {d\mspace{14mu}{and}\mspace{14mu}{lsnr}} > e_{2}} \\ \beta_{2} & {\varphi \geq {d\mspace{14mu}{and}\mspace{14mu} e_{1}} < {lsnr} \leq e_{2}} \\ \beta_{3} & {\varphi \geq {d\mspace{14mu}{and}\mspace{14mu}{lsnr}} \leq e_{1}} \\ \beta_{4} & {otherwise} \end{matrix} \right.}$ wherein β₁<β₂<β₃ and β₄ and d as well as e₁<e₂ are integer or floating numbers and wherein lsnr is the long term SNR of the input audio signal.

In a possible specific implementation, β₁=4β₂=10, β₃=15 and β₄=9. In this specific implementation, d is set to 1, and e₁=8 and e₂=18.

The modified segmental SNR (msnr) is derived in step S5 by adding the sub-band's specific parameters (sbsp). In a further step S6 of the implementation of the method for adaptively detecting a voice activity in an input audio signal as shown in FIG. 1, the derived modified segmental SNR (mssnr) is compared with a threshold value thr to provide a VAD decision (VADD).

In a possible implementation, the derived modified segmental SNR (mssnr) is compared with a threshold value thr which is set to:

${thr} = \left\{ \begin{matrix} k_{1} & {{lsnr} > e_{2}} \\ k_{2} & {e_{1} < {lsnr} \leq e_{2}} \\ k_{3} & {{lsnr} \leq e_{1}} \end{matrix} \right.$ wherein k₁>k₂>k₃ and e₁<e₂ are integer or floating numbers, and wherein the VAD decision (VADD) is generated as follows:

${V\; A\; D\; D} = \left\{ \begin{matrix} 1 & {{mssnr} > {thr}} \\ 0 & {{mssnr} \leq {thr}} \end{matrix} \right.$ wherein VADD=1 indicates an active frame with voice activity being present, and VADD=0 indicates a passive frame with voice activity being absent.

In a possible specific implementation, k₁=135, k₂=35, k₃=10 and e₁ is set to 8 whereas e₂ is set to 18.

In a further possible implementation of the method for adaptively detecting a voice activity in an input audio signal, the first variable α of the adaptive function (AF) may be calculated by: α=g(i, lsnr,ε)=a(i)lsnr+b(i)+c(ε) wherein a(i), b(i) are real numbers depending on a sub-band index i, and c(ε) is a real number depending on the estimated fluctuation of the background noise of the input audio signal, and wherein the second variable β of the adaptive function (AF) may be calculated by:

$\beta = {{h\left( {{lsnr},\varphi,ɛ} \right)} = \left\{ \begin{matrix} \beta_{1} & {\varphi \geq {d\mspace{14mu}{and}\mspace{14mu}{lsnr}} > {e_{2}\mspace{14mu}{and}\mspace{14mu} ɛ} \leq p} \\ \beta_{2} & {\varphi \geq {d\mspace{14mu}{and}\mspace{14mu}{lsnr}} > {e_{2}\mspace{14mu}{and}\mspace{14mu} ɛ} > p} \\ \beta_{3} & {\varphi \geq {d\mspace{14mu}{and}\mspace{14mu} e_{1}} < {lsnr} < {e_{2}\mspace{14mu}{and}\mspace{14mu} ɛ} \leq p} \\ \beta_{4} & {\varphi \geq {d\mspace{14mu}{and}\mspace{14mu} e_{1}} < {lsnr} < {e_{2}\mspace{14mu}{and}\mspace{14mu} ɛ} > p} \\ \beta_{5} & {\varphi \geq {d\mspace{14mu}{and}\mspace{14mu}{lsnr}} \leq {e_{1}\mspace{14mu}{and}\mspace{14mu} ɛ} \leq p} \\ \beta_{6} & {\varphi \geq {d\mspace{14mu}{and}\mspace{14mu}{lsnr}} \leq {e_{1}\mspace{14mu}{and}\mspace{14mu} ɛ} > p} \\ \beta_{4} & {\varphi < d} \end{matrix} \right.}$

-   -   wherein φ=f(snr (i))+α and ε is the estimated fluctuation of the         background noise and d and e₁<e₂ and p are integer or floating         numbers.

In a specific implementation the parameters are set as follows: β₁=3, β₂=4, β₃=7, β₄=10, β₅=8, β₆=15, β₇=15, and d=1, e ₁=8, e ₂=18, p=40.

In an implementation of the method adaptively detecting a voice activity in an input audio signal according to the first aspect of the present disclosure, the derived modified segmental SNR (mssnr) is compared with a threshold value (thr) being set to:

${thr} = \left\{ \begin{matrix} {q_{1} + {r_{1} \cdot {{Min}\left\lbrack {\frac{{lsnr} - v_{1}}{W_{1}},1} \right\rbrack}}} & {{lsnr} > e_{2}} \\ {q_{2} + {r_{2} \cdot {{Min}\left\lbrack {\frac{{lsnr} - v_{2}}{W_{2}},1} \right\rbrack}}} & {e_{1} < {lsnr} \leq e_{2}} \\ {q_{3} + {r_{3} \cdot {{Max}\left\lbrack {\frac{{lsnr} - v_{3}}{W_{3}},1} \right\rbrack}}} & {{lsnr} \leq e_{1}} \end{matrix} \right.$ wherein q₁, q₂, q₃ and r₁, r₂, r₃ and e₁<e₂ and v₁, v₂, v₃ and W₁, W₂, W₃ are integer or floating numbers.

In a specific implementation of the first aspect of the present disclosure, q₁=20, q₂=30, q₃=9 and r₁=30, r₂=10 and r₃=2. Further, v₁=18, v₂=8 and v₃=5 and W₁=8, W₂=10 and W₃=3. Further, the parameters e₁, e₂ are set to e₁=8 and e₂=18.

Accordingly, in a possible embodiment, not only a background noise estimation and a long term SNR estimation is performed but additionally also a background noise variation estimation is performed to determine a background noise fluctuation ε of the background noise of the input audio signal.

Two factors, α, β of the adaptive function (AF) adjust a trade-off of the discriminating power of the modified segmental SNR parameter. Different trade-offs signify that the detection is more favorable for either active or inactive detection for the received frames. Generally the higher the long term SNR of the input audio signal is the more favorable it is to adjust the modified segmental SNR for active detection by means of adjusting the corresponding coefficients α, β of the adaptive function.

The VAD decision performed in step S6 can further go through a hard hang-over procedure. A hard hang-over procedure forces the VAD decisions for several frames to be active immediately after the VAD decision obtained in step S6 changes from active to inactive.

In a possible implementation of the method for adaptively detecting a voice activity in an input audio signal according to the first aspect of the present disclosure, the background noise of the input audio signal is analyzed and a number representing the extent of stationarity or fluctuation of the background noise, denoted by ε, is generated. This fluctuation ε of the background noise can be calculated, for example, by: ε=ω·ε+(1−ω)·ssnr_(n) wherein ω is a forgetting factor usually between 0.9-0.99 and ssnr_(n) is the summation of snr(i) over all sub-bands of the frame detected as a background frame multiplied by a factor of e.g. 10.

FIG. 2 shows a block diagram of a VAD apparatus 1 according to a second aspect of the present disclosure. The VAD apparatus 1 comprises a SNR based VAD parameter calculation unit 2 receiving an input audio signal applied to an input 3 of the VAD apparatus 1. The SNR based VAD parameter calculation unit 2 calculates a SNR to each sub-band of an applied input frame of the input audio signal and a sub-band's specific parameter based on the respective sub-band SNR for each sub-band using an adaptive function and derives a modified segmental SNR by adding the sub-band's specific parameters. The derived modified segmental SNR is applied by the SNR based VAD parameter calculation unit 2 to a VAD decision generation unit 4 of the VAD apparatus 1. The VAD decision generation unit 4 generates a VAD decision (VADD) by comparing the modified segmental SNR with a threshold value. The generated VAD decision (VADD) is output by the VAD apparatus 1 at an output 5.

In a possible implementation of the VAD apparatus 1 according to the second aspect of the present disclosure, the VAD detection apparatus 1 further comprises a noise characteristic determination unit 6 as shown in FIG. 2. The noise characteristic determination unit 6 determines a noise characteristic of the input signal based at least on a received input frame of the input audio signal applied to input 3 of the VAD apparatus 1. In an alternative implementation the noise characteristic is applied to the SNR based VAD parameter calculation unit 2 from an external noise characteristic determination entity. In a possible implementation of the VAD apparatus 1 according to the second aspect of the present disclosure the noise characteristic determination unit 6 as shown in FIG. 2 can comprise a long term SNR estimation unit which calculates a long term SNR of the input audio signal. In a further possible implementation, the noise characteristic determination unit 6 can also comprise a background noise variation estimation unit which calculates a stationarity or fluctuation ε of the background noise of the input audio signal. Accordingly, the noise characteristic provided by the noise characteristic determination unit 6 can comprise a long term SNR of the input audio signal and/or a stationarity or fluctuation of the background noise of the input audio signal. In a possible implementation an adaptive function used by the SNR based VAD parameter calculation unit 2 is selected dependent on at least one noise characteristic determined by the noise characteristic determination unit 6.

FIG. 3 shows a block diagram of an audio signal processing device 7 according to a third aspect of the present disclosure. The signal processing device 7 comprises the VAD apparatus 1 providing a VAD decision (VADD) for an audio signal processing unit 8 within the audio signal processing device 7. The audio signal processing of an input audio signal is performed by the audio signal processing unit 8 depending on the received VAD decision (VADD) generated by the VAD apparatus 1 according to the first aspect of the present disclosure. The audio signal processing unit 8 can perform for example an encoding of the input audio signal based on the VAD decision (VADD). The audio signal processing device 7 can form part of a speech communication device such as a mobile phone. Further, the audio signal processing device 7 can be provided within a speech communication system such as an audio conferencing system, an echo signal cancellation system, a speech noise reduction system, a speech recognition system or a speech encoding system. The VADD generated by the VAD apparatus 1 can control in a possible implementation a discontinuous transmission DTX mode of an entity, for example an entity in an cellular radio system, for example a GSM or LTE or CDMA system. The VAD apparatus 1 can enhance the system capacity of a system such as a cellular radio system by reducing co-channel interferences. Furthermore, the power consumption of a portable digital device within a cellular radio system can be reduced significantly. 

What is claimed is:
 1. An audio signal encoding method used by a signal processing apparatus comprising a processor and a memory, the method comprising: obtaining a frame of an audio signal, wherein the frame comprises a plurality of sub-bands; determining a long term signal to noise ratio of the audio signal; obtaining a signal to noise ratio of each frequency sub-band of the frame; calculating a sub-band specific parameter (sbsp) of each frequency sub-band using an adaptive function, wherein the sbsp of the i^(th) frequency sub-band sbsp(i) is calculated as follows: sbsp(i)=(f(snr(i))+α)^(β) where i is a sub-band index of the i^(th) frequency sub-band, snr(i) is the signal to noise ratio of the i^(th) frequency sub-band, (f(snr(i))+α)^(β) is the adaptive function, α is determined based on the sub-band index i and the long term signal to noise ratio of the audio signal, and β is a configurable variable; obtaining a modified segmental signal to noise ratio (mssnr) by summing up the calculated sbsp of each frequency sub-band; comparing the mssnr with a threshold value (thr) to provide a voice activity detection decision (VADD); and encoding the audio signal based on the VADD.
 2. The method according to claim 1, wherein α=g(i, lsnr), where g(i, lsnr) is a linear or non-linear function.
 3. The method according to claim 2, wherein α=g(i, lnsr)=α(i)×lsnr+b(i) wherein a(i) and b(i) are values depending on the sub-band index i.
 4. The method according to claim 1, wherein β is determined based on the long term signal to noise ratio of the audio signal.
 5. The method according to claim 1, wherein determining a long term signal to noise ratio of the audio signal comprises: determining the long term signal to noise ratio of the audio signal based at least on the frame of the audio signal.
 6. The method according to claim 1, wherein obtaining a signal to noise ratio of each frequency sub-band comprises: obtaining a signal energy of the frequency sub-band; estimating a background noise energy of the frequency sub-band; and calculating the signal to noise ratio of the frequency sub-band based on the signal energy and the background noise energy of the frequency sub-band.
 7. The method according to claim 6, wherein the signal energy of the frequency sub-band is a smoothed signal energy, and wherein the smoothed signal energy is a weighted result of a signal energy of the frequency sub-band in a currently received frame and the signal energy of the frequency sub-band in at least one previously received frame.
 8. The method according to claim 1, wherein comparing the mssnr with a threshold value (thr) to provide a voice activity detection decision (VADD) comprises: generating the VADD according to: ${V\; A\; D\; D} = \left\{ \begin{matrix} 1 & {{mssnr} > {thr}} \\ 0 & {{mssnr} \leq {thr}} \end{matrix} \right.$ wherein VADD=1 indicates that the voice activity is present in the frame of the audio signal, and VADD=0 indicates that the voice activity is absent in the frame of the audio signal.
 9. An audio signal processing apparatus, comprising: one or more processors and a memory configured to store program instructions for execution by the processors; wherein the one or more processors, by executing the program instructions, are configured to: obtain a frame of an audio signal, wherein the frame comprises a plurality of sub-bands; determine a long term signal to noise ratio of the audio signal; obtain a signal to noise ratio of each frequency sub-band of the frame; calculate a sub-band specific parameter (sbsp) of each of N frequency sub-bands using an adaptive function, wherein the sbsp of the i^(th) frequency sub-band sbsp(i) is calculated as follows: sbsp(i)=(f(snr(i))+α)^(β) where i is a sub-band index of the i^(th) frequency sub-band, snr(i) is the signal to noise ratio of the i^(th) frequency sub-band, (f(snr(i))+α)^(β) is the adaptive function, α is determined based on the sub-band index i and the long term signal to noise ratio of the audio signal, and β is a configurable variable; obtain a modified segmental signal to noise ratio (mssnr) by summing up the calculated sbsp of each frequency sub-band; compare the mssnr with a threshold value (thr) to provide a voice activity detection decision (VADD); and encode the audio signal based on the VADD.
 10. The apparatus according to claim 9, wherein α=g(i, lsnr), where g(i, lsnr) is a linear or non-linear function.
 11. The apparatus according to claim 10, wherein α=g(i, lnsr)=α(i)×lsnr+b(i) wherein a(i) and b(i) are values depending on the sub-band index i.
 12. The apparatus according to claim 9, wherein β is determined based on the long term signal to noise ratio of the audio signal.
 13. The apparatus according to claim 9, wherein in determining a long term signal to noise ratio of the audio signal, the one or more processors are configured to: determine the long term signal to noise ratio of the audio signal based at least on the frame of the audio signal.
 14. The apparatus according to claim 9, wherein in obtaining a signal to noise ratio of frequency sub-band, the one or more processors are configured to: obtain a signal energy of the frequency sub-band; estimate a background noise energy of the frequency sub-band; and calculate the signal to noise ratio of the frequency sub-band based on the signal energy and the background noise energy of the frequency sub-band.
 15. The apparatus according to claim 14, wherein the signal energy of the frequency sub-band is a smoothed signal energy, and wherein the smoothed signal energy is a weighted result of a signal energy of the frequency sub-band in a currently received frame and the signal energy of the frequency sub-band in at least one previously received frame.
 16. The apparatus according to claim 9, wherein in comparing the mssnr with a threshold value (thr) to provide a voice activity detection decision (VADD), the one or more processors are configured to: generate the VADD according to: ${V\; A\; D\; D} = \left\{ \begin{matrix} 1 & {{mssnr} > {thr}} \\ 0 & {{mssnr} \leq {thr}} \end{matrix} \right.$ wherein VADD=1 indicates that the voice activity is present in the received frame of the audio signal, and VADD=0 indicates that the voice activity is absent in the frame of the audio signal. 